Behavioral interaction between licitly and illicitly abused drugs will be studied in human volunteers. Within a residentail research setting volunteers will be permitted to self-administer nicotine by smoking cigarettes. An initial experiment will examine the effects of response-cost manipulations on nicotine self-administration. Two studies will explore the phenomenon that ethanol facilitates cigarette smoking by determining whether this effect is specifically dependent upon nicotine. Three other experiments will examine the effects of sodium pentobarbital, d-amphetamine sulfate, and caffeine on nicotine self-administration. Throughout the studies, the self-administration behavior will be monitored by counting the number of cigarettes smoked, and by measuring the frequency, size and duration of individual cigarette puffs with a specially constructed device. A carbon monoxide analyzer will be used to provide a physiological measure of amount of cigarette smoking. Overall, these studies should provide an improved understanding of the addictive process by describing the interactive relations among commonly abused drugs.